U.S. Pat. No. 4,105,776, issued Aug. 8, 1978 to Miguel Angel Ondetti and David W. Cushman, and its parent U.S. Pat. No. 4,046,889, issued Sept. 6, 1977, disclose certain mercaptoacyl derivatives of the naturally occurring amino acids proline and hydroxyproline which are angiotensin converting enzyme inhibitors and can be used for the reduction of blood pressure. It has now been found that certain synthetic hydroxyproline derivatives, wherein the pyrrolidine ring of proline bears a carbamate group, also provide new chemical compounds which have utility as hypotensive agents.